1. Field of the Invention
This invention relates to electric machines working as motors or generators. The invention is particularly applicable to reluctance motors.
2. Background Art
FIG. 1 of the drawings illustrates a known construction of single-phase salient pole switched reluctance machine which can be operated as either an electric motor or generator. However, for the sake of convenience the term motor will be used throughout this specification though it is to be understood that the invention is equally applicable to electric generators.
The switched reluctance motor of FIG. 1 comprises a stator core 1 and a rotor 2 which are both formed with four salient poles 3 and 4, respectively. The stator core is made up of numerous metal lateral laminations in a stack. Each salient stator pole 3 carries an exciting coil 5. The coils 5 are wound such that North (N) and South (S) poles are created alternately around the stator. As illustrated by the broken lines 6, each sector of the assembled laminations of the core 1 carries half the flux created in one of the associated adjacent stator poles.
In this kind of motor the laminations are usually stamped from a blank of strip metal. As the general outer shape of each lamination will be circular there will be a significant amount of material left between stampings of laminations which can only be cast aside as scrap metal. Although this scrap can, of course, be recycled, it is clearly desirable to utilise the strip with as little waste as possible.
Furthermore, each pole carries its own copper winding. This construction requires a considerable amount of copper which will add both to the weight and cost of the motor.
It is an object of the present invention to provide an electric machine which optimises the use of the core material and the copper windings in order to reduce the cost of manufacture.
According to the present invention there is provided an electric reluctance machine comprising a primary having a primary core defining primary poles; a secondary having secondary poles; and a coil or set of coils, the or each coil being connected and/or being located with respect to the primary poles so that the relative magnetic orientation of those poles in a group of adjacent primary poles with respect to the coil or coils is the same and the said orientation of the groups of primary poles is different in adjacent groups, wherein a portion of the primary core between adjacent primary poles in a group is relatively radially thinner than that between adjacent groups of primary poles.
Thus, there; is substantially no flux between poles of like polarity and this lack of flux allows the material of the core between like poles to be reduced accordingly.
The terms primary and secondary are used to denote the relatively moving parts of the machine. Clearly, the poles associated with the coil or coils could be the rotor or stator of the machine.
In this way the flux through the core between the groups of stator poles is increased by an amount related to the number of stator poles in a group. Also, the flux in the stator core between poles within a group is substantially reduced toward the centre of the group. Hence that portion of the stator core between the stator poles in a group is now less dependent on considerations of the amount of flux in it as it no longer has to serve as a path for all the flux between dissimilar groups of poles. Thus, the thickness of the stator core in this region can be reduced and even shaped and remoulded with less regard to the flux in it. The main remaining criterion lies in providing sufficient mechanical strength for the stator core. However, a slight increase in thickness of the material of the stator core will probably be required between the dissimilar groups due to the increased flux in that region.
Preferably, each group of adjacent stator poles is embraced by a single coil. Thus, the amount of material, e.g. copper, needed for the windings in a given machine will be reduced.
Preferably, the thinner portion of the stator core defines a flat outer face of the core.
Preferably, the core is made up of a plurality of laterally extending laminations. In this case when the flat outer faces are formed of the thinner portions of the core the laminations can be stamped from a strip so as to use less material to form the laminations and have less waste material left over from the stampings. In this latter regard if the thinner or flat portions are stamped adjacent one another in a line from a strip, the overall length of the strip used can also be reduced as well as leaving less scrap.
The invention also extends to a machine as described above connected to run as a motor and the same machine connected to run as a generator.